In previous patent applications namely U.S. patent application Ser. No. 08/644,978 filed May 15, 1996 and PCT application PCT/CA/96/00879 filed Dec. 24 1996, methods have been disclosed for the manufacture of high density powder metal articles that may contain up to 0.5% of carbon by weight. In some applications, a higher carbon content is desirable, together with high density in order to achieve specific mechanical property requirements.
Because carbon additions to iron increase hardness and reduce ductility, high density forming of higher carbon materials is usually not practical. However, in this invention, a method has been determined whereby combined selection of material composition, and thermal processing methods, can produce high carbon materials of a formable nature. High carbon materials processed in the prescribed manner to be described herein are of significantly lower hardness than usually expected, and offer advantageous forming characteristics that can be used to produce high density powder metal articles.
Forming as defined herein includes:
(a) sizing--which may be defined as a final pressing of a sintered compact to secure a desired size or dimension; PA1 (b) coining--which can be defined as pressing a sintered compact to obtain a definite surface configuration; PA1 (c) repressing--which can be defined as the application of pressure to a previously pressed and sintered compact, usually for the purpose of improving physical or mechanical properties and dimensional characteristics; PA1 (d) restriking--additional compacting of a sintered compact.
In carbon containing powder metal steels that are processed in the normal manner, FIG. 1 illustrates that as carbon content is raised, the density achieved on cold forming is significantly reduced. For example, FIG. 1 shows that at the 60 tons per square inch forming pressure, a sintered part with 0.2% carbon, a density of approximately 7.5 g/cc would be achieved. With 0.6% carbon, at the same forming pressure, a density of only 7.3 g/cc would be achieved.
It is an object of this invention to provide an improved method to produce powder metal parts having improved formability at higher carbon contents.
This invention details methods of processing high carbon materials in a manner that minimizes the above reduction in formability that is usually experienced at higher carbon contents.